View Full Version : 737 NG Bleed Trips

2nd May 2003, 22:02
At my company we seem to be experiencing an inordinate number of right side Bleed Trips at "top of descent".
Are any of you experiencing the same?
Have you had the problem and overcome it?
If your company had or is experiencing the problem currently, do you find one side to be more likely to trip than the other?
It's driving our 25 year old engineers crazy...
The older heads' attitude is tell thel drivers not to pull the throttles back so far so fast and the thing won't trip, or, let the FMC and Autothrottle do the job.

3rd May 2003, 07:48
Please don't confuse Bleed Trips with Pack Trips.

Bleed Trips are triggered by high temperature or high pressure in the Bleed Ducts downstream of the engine (measured in engine pylon). The only air this single heat exchanger feels is engine bleed air so any object plugging this would have to go thru the engine core. A journey it is not likely to survive or not leave a permanent mark on the fan. These trips are MELable with major operating restrictions, ie "no known or forecast icing, and single pack ops only. These Bleed Trips can be reduced to single events if proper T-shooting is followed. This requires special measurment tools and time so that part is easier said than accomplished. Once the shotgun is broken out and the suspect parts are changed in inverse diffulculty order you (the operator) are doomed to many nuisance trips.

Pack Trips can be caused by many faults but plugged heat exchangers are not usually one of them. Each pack has two heat exchangers and the inlet for these is on the under belly just aft of the forward cargo door. this is one with major airflow into it when you are going under the wing to check the tires. These heat exchangers can be plugged by airport environment trash but are usually just dirty from dust, dirt and I suspect primarily "SMOG". If one of these heat exchangers is plugged you will usually see it as "Ram Air Door Full Open" light on at top of descent. this is also fully MELable with minimal restrictions.

If you are in a position to influence your "25 year old engineers" than encourage them and their management to fully follow the Boeing FIM before shotguning the easy parts. This will require management to schedule the airplane into maintenance where the people, tools and most importantly the time are available.


3rd May 2003, 09:01
Have no fear I and even the kids on the third floor know the difference. As for Heat Exchangers and Precoolers, many people confuse them. They do, after all, reduce the heat in a given body of air. Of course if there were any of the items he mentioned in the precooler I'm sure the Fan Blades would look pretty gruesome.
I'm talking about Bleed Trips at top of descent.
And I know the rest of it too. That's why I came here to see if there's anyone out there with any information that I can't pry out officially out of people in other companies operating 800's.
I've exhausted all my good-ol'-boy sources at other companies.:}

3rd May 2003, 17:21
Ask if the bi-metallic strip in the pre-cooler temp controller has been cleaned(outlet for the precooler) and/or the filter in the high/low bleed control valve checked for crud.As eskimo has pointed out high temp (by the first)and high pressure (by the later)

4th May 2003, 22:28
Never had this problem, and we use idle (in general) from TOD. I've operated NG's in ground temperatures ranging from -36 to + 42 degrees Celsius. Artic, desert and tropical environments.
Sorry, can't help you much, the air systems have been working superbly all the time.

Maintenance procedures?

5th May 2003, 11:41
This is the second forum that this topic has been posted in without much feedback from you.
What type of B737 Next Gen is it, -600/700 or -800/900??
What has been replaced??

Yes there are 2 ways to cause a bleed trip, overtemperature and overpressure, but if I may offer my 2 cents...if this isn't some kind of "gremlin", it seems more likely that it would be some kind of overpressure problem.

An actual overtemperature bleed trip means that the precooler system failed (or did its job but couldn't lower the temp) AND/OR the 450 degree thermostat failed (or did it's job).
Its would be helpful to know if the pressure gradually decreased or just shut off.

6th May 2003, 03:43
Well, You've all had the same ideas we have and I appreciate it greatly. Thank you.

The reason I came to this forum is to see if there might be something we're missing or if anyone else has had a similiar problem that the engineering or maint departments don't want to admit. The anonymity here can be a great advantage.

Ours are all -800's and it happens on a number of different planes. In one case we did find a chafing wire bundle and there's a service bulletin out on that now which we're actively employing.

The trips seem to all reset after a short time which indicates a temp trip. My personal theory is that since there's no anticipation in the system like in the DAC planes, when the engine comes to an abrupt idle after high altitude cruising, the 9th stage opens up to compensate for the sudden lack of air and brings with it hotter air tripping the 490. Once things stabilize the system resets just fine. That's a view shared with some of my friends up north who, for professional reasons, prefer to remain anonymous.

I have heard from one of my old comrades-in-arms at a competitor who tells me that three years ago, before Boeing changed the software raising the flight idle RPM, they were instructed to pull back to 50%, let the things stabilize, then pull the rest of the way back to idle (these guys don't have autothrottle). Everyone else I've talked to tells me that when using the autothrottle there's no problems since the software upgrade.

We do have one chronic bad actor and we've even changed the engine, (for unrelated reasons) all the valves (because it kept tripping), plus the wiring harnesses, and it still trips. I want to go after the precooler but that's just my own personal brand of kneejerk shotgun troubleshooting.

My other idea is to see if it's tied to one group who may simply be pulling the throttles back like some used to do in DC-8's to see how many noise complaints could be logged under the approach path into ATL on a cold winter's morn. (Nothing can backfire like a short duct JT-3)

Once again, thanks all very much for the input. Keep it coming...

6th May 2003, 05:38
Didn't know you could get the NG without autothrottle.
Anyway, I've flown -800's on 10 minute sectors as well as 5 hour sectors. 90% of the time we use VNAV or LVL CHG from TOD, so the autothrottle will retard to idle from the very start.
Again, no problems.
If operating with manual throttles I don't really think you would retard the throttles any faster than the autothrottle rate. This would be very uncomfortable in the back. Then again, I have met plenty of pilots who have "seen the light", and operate according to logic understood only by themselves.
Nothing is impossible.